Tunable laser systems are used to measure wavelength-dependent characteristics in a variety of test applications. For example, a tunable laser system may be used to make wavelength-dependent loss or dispersion measurements on a device under test (DUT), such as an optical component. To attain higher measurement speeds, the laser output of the tunable laser is generally swept over a wavelength range. In each sweep, the wavelength of the laser output varies as a function of time between a start wavelength and a stop wavelength in the wavelength range.
A conventional tunable laser system, typically, includes a single short-period filter, such as a fiber etalon. The filter provides a series of pulses from a portion of the laser output, in which the pulses are separated in wavelength by the free spectral range (FSR) of the filter. In order to determine the instantaneous wavelength of the laser output, the pulses in the series of pulses are counted. Thus, the series of pulses provides fine wavelength discrimination at a wavelength resolution determined by the FSR of the filter.
However, the starting wavelength accuracy of the tunable laser is, generally, much lower than the wavelength resolution. In other words, the tunable laser cannot reliably begin a sweep at a start wavelength within a period of the filter. Therefore, a pulse count only provides the relative wavelength of the laser output at a particular point in time. To determine the absolute wavelength of the laser output at the particular point in time, the pulse count must be related to a wavelength reference. Typically, one or more reference lines from a reference cell, e.g., a gas absorption cell, are used for wavelength referencing. Unfortunately, the reference lines provided by a reference cell are, generally, limited in number and in wavelength.
Furthermore, determining the instantaneous wavelength of the laser output by counting the pulses in the series of pulses is only accurate if the wavelength of the laser output varies continuously over the wavelength range. However, the tunable laser, especially near the end of its life, may undergo one or more mode hops during a sweep. When a mode hop occurs, the wavelength of the laser output jumps by a wavelength interval that is a function of the cavity length of the tunable laser. Unfortunately, since the series of pulses provided by the filter is periodic in wavelength, the conventional tunable laser system cannot detect such a wavelength discontinuity.
In attempts to overcome these problems, tunable laser systems including a long-period filter, in addition to a short-period filter, have been developed. A series of pulses provided by the short-period filter provides fine wavelength discrimination, and a series of pulses provided by the long-period filter provides coarse wavelength discrimination. As described in U.S. Pat. No. 6,795,196, issued on Sep. 21, 2004 to Funakawa, which is incorporated herein by reference, the series of pulses provided by the long-period filter may be used for wavelength referencing. As described in U.S. Pat. No. 7,079,253, issued on Jul. 18, 2006 to North-Morris, et al., which is incorporated herein by reference, the series of pulses provided by the long-period filter may be used for mode-hop detection.
However, in general, the short-period filter and the long-period filter in these tunable laser systems are different types of filter. For example, the short-period filter may be a fiber etalon, whereas the long-period filter may be a more expensive bulk-glass etalon or air-gap etalon. Typically, the short-period filter and the long-period filter have different dispersion and temperature characteristics, which may lead to a drift in the relationship between the FSRs of the two filters. Therefore, a tunable laser system including two short-period filters, which may be selected to be the same type of filter and to have matched characteristics, would be more desirable.
Tunable laser systems including two short-period filters having the same FSR, but opposite slopes are described in U.S. Pat. No. 6,359,685, issued on Mar. 19, 2002 to Colbourne, et al., and in U.S. Pat. No. 6,061,124, issued on May 9, 2000 to Nyman, et al., which are incorporated herein by reference. In these tunable laser systems, two series of pulses provided by the two short-period filters are differentially amplified to provide an electrical signal, which is used to encode the laser output of the tunable laser with wavelength information.